1. Field of the Invention
The present invention is related to a calibration circuit and a frequency generator, and particularly to a calibration circuit that can calibrate an oscillation frequency of a crystal oscillator according to a temperature, and a frequency generator that can stably output a frequency not influenced by the temperature.
2. Description of the Prior Art
Quartz is a naturally piezoelectric material. When an external voltage is applied to upper and lower sides of the quartz, the quartz generates mechanical deformation due to a coupling effect between mechanical and electrical characteristics of the quartz, resulting in a voltage drop being generated across the upper and lower sides of the quartz. If an alternating voltage is applied to the upper and lower sides of the quartz, the quartz can generate periodic oscillations. A crystal oscillator composed of quartz has an advantage of high frequency stability, so the crystal oscillator can act as a standard frequency source in various applications, including remote communications, mobile telephone systems, global positioning systems, navigation, remote controls, aerospace industries, high-speed computers, precision measurement instruments, consumer electronics, and other fields for providing accurate frequencies.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a frequency f generated by the crystal oscillator varying with a temperature. As shown in FIG. 1, the frequency f generated by the crystal oscillator shifts slightly from a target frequency f0 with the temperature, where a vertical axis in FIG. 1 is a ratio of a difference Δf between the frequency f and the target frequency f0 to the target frequency f0. Therefore, the crystal oscillator needs a calibration circuit to compensate for a frequency error of the crystal oscillator that varies with the temperature.